# filter

* [集合的filter操作](https://www.selinux.tech/scala/function-operation/pages/-Lrj-CnUKrtgc-lsO2HT#集合的filter操作)
  * [函数的定义](https://www.selinux.tech/scala/function-operation/pages/-Lrj-CnUKrtgc-lsO2HT#函数的定义)
  * [作用](https://www.selinux.tech/scala/function-operation/pages/-Lrj-CnUKrtgc-lsO2HT#作用)

## 函数的定义

首先以 Seq 的filter函数定义来进行理解。

```scala
def filter(p: (A) ⇒ Boolean): Seq[A]
  Selects all elements of this traversable collection which satisfy a predicate.

  p
  the predicate used to test elements.

  returns
  a new traversable collection consisting of all elements of this traversable collection that satisfy the given predicate p. The order of the elements is preserved.
```

## 作用

将集合中符合条件的 元素 筛选出来，放入到一个新的集合中，并返回这个新的集合。 这些操作，对原来的集合是没有影响的。

```scala
object FilterDemo1 {

  def main(args: Array[String]): Unit = {
    val list = List("Tom", "Aerry", "Jack").filter(startA)

    println(list)

  }

  def startA(string: String):Boolean ={
    string.startsWith("A")
  }

}
```

```scala
object FilterDemo2 {

  def main(args: Array[String]): Unit = {
    val list = List("Tom", "Aerry", "Jack").filter(_.startsWith("A"))

    println(list)
  }

}
```


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